Effects of thermal conduction and natural convection on directional melting characteristics of paraffin

At present, electronic devices are increasingly prone to severe heat dissipation problems owing to higher integration and miniaturization. Generally, the performances and service lives of electronic devices both decrease with increase in the working temperatures. In contrast, the failure rates of electronic devices rapidly increase when the working temperature increases gradually. For instance, the working temperatures of insulated gate bipolar transistor (IGBT) modules and missile-borne radar systems should be maintained below 150 degrees C. Moreover, the heat fluxes of these systems present transient and intermittent characteristics. This indicates that the traditional finned radiators are increasingly failing to meet the cooling demands of these systems. Generally, the melting process of phase change materials (PCMs) requires a nearly constant temperature (or limited within a temperature range) and absorbs a large amount of latent heat; this hints that the PCM heat sink may possibly help prevent high-power thermal shock and ensure safe operation of electronic devices with variable heat fluxes. To further improve the thermal shock resistances of PCM heat sinks, an experimental system (consisting of a heating unit, an experiment unit, and a measuring unit) was established to explore the directional melting characteristics of paraffin under four conditions (thermal conduction and natural convection, thermal conduction, thermal conduction enhancement and natural convection suppression, and thermal conduction enhancement) by changing the heating positions and adding copper foam. Specifically, paraffin with a phase change temperature of 56.9-63.6 degrees C was adopted as the working medium. Moreover, copper foam of dimensions 58 mm x 20 mm x 95 mm and porosity 15 PPI was used to enhance the heat conduction performance of paraffin. It should be noted here that the natural convection in the melting process of paraffin is driven by buoyancy, i.e., natural convection can be ignored when paraffin is heated from the top. Besides, the maximum thermal loss and maximum uncertainty error of this experimental system were 2% and 1.6%, respectively. These results indicate that the inside temperature of pure paraffin successively experiences a slow increase, rapid rise, slight fluctuation, and then gradual increase when the thermal conduction and natural convection concurrently affect the melting process. By contrast, these four stages of temperature evolution change to gradual increase, linear rise, remaining constant, and continued increase once the temperature gradient inside the paraffin significantly decreases owing to the copper foam. The protection time of the PCM heat sink is prolonged from 483 s to 4400 s once natural convection is used for the paraffin melting process. Generally, it can be considered that adding copper foam is an effective method of smoothing the temperature gradient inside paraffin. Meanwhile, the copper foam can significantly prolong the manifestation, shorten the duration, and weaken the influence of natural convection. Specifically, the protective time is only prolonged by 80 s owing to the decrease of the influence ratio of natural convection to thermal conduction from 8.1 to 0.72. Therefore, the natural convection of PCMs should be preferentially strengthened for electronic devices with low heat fluxes and high working temperatures, whereas the thermal conductivity should be appropriately enhanced with increasing heat fluxes.

Automated summary

The study indicates that using PCM heat sink can effectively reduce the risk of thermal shock to electronic devices and ensure safe operation. Experimental results show that by changing heating positions and adding copper foam, the thermal shock resistance of PCM heat sink can be significantly improved. By using copper foam, the temperature gradient inside paraffin can be smoothed, prolonging the protection time of the heat sink.